Image display devices such as cathode ray tube (CRT) displays, liquid crystal displays (LCDs), plasma displays (PDPs), electroluminescence displays (ELDs), electronic paper, tablet PCs, and touch panels typically have, on their outermost surface, an optical layered body for anti-reflection. Those optical layered bodies for anti-reflection suppress reflection of an image or reduce the reflectance by diffusing or interfering light.
Those optical layered bodies for anti-reflection are formed through a dry process such as deposition or sputtering by, for example, a method of forming a thin film of a substance having a low refractive index (e.g. MgF2) on a base film, or a method of forming alternating layers of a substance having a high refractive index [e.g. ITO (tin-doped indium oxide, TiO2] and a substance having a low refractive index (e.g. MgF2, SiO2) on a base film.
Anti-reflection films produced through such a dry process, however, have a problem of a high production cost.
To solve the problem, a wet process, i.e., production of optical layered bodies for anti-reflection by coating, has recently been attempted.
For example, Patent Literature 1 describes an optical film that has a transparent substrate, a hard coat layer disposed on the substrate, and a low-refractive-index layer having a specific reflective index disposed on the hard coat layer.
In production of such an optical film, a typical method of providing functional layers on a substrate is the sequential multilayer coating in which coating and drying of a layer are repeated by, for example, roll coating (e.g. reverse roll coating, gravure roll coating), blade coating, wire bar coating, or die coating.
Also, Patent Literature 2, for example, discloses lamination of layers with different refractive indexes as a method for reducing reflection of light emitted from an external light source such as a fluorescent lamp and for eliminating interference fringes to increase the visibility of the display.
However, such a sequential lamination method repeats the application and drying process for multiple times, and thus leads to relatively many occasions for the layers to be exposed to the air, which sometimes causes deterioration of the functions or defects resulting from external foreign matters. Also, since the heat-drying process is repeated for multiple times, the productivity is unfortunately unfavorable in terms of the utilization efficiency of energy, and an increase in the number of layers results in an increase in the number of steps, complicating the production and increasing the production cost.
There is also a problem that the above low-refractive-index layers are thin films formed by application of compositions in the state of having a low solids content, which causes a low application rate to lead to a low productivity. Furthermore, the adhesion between the layers is low to give inferior durability.
A known method to solve these problems is producing an optical film by simultaneous lamination of at least two layers as described in, for example, Patent Literature 3, which includes laminating at least two functional layers at the same time, subjecting the layers to first ionizing radiation exposure, drying the layers, and subjecting the layers to second ionizing radiation exposure for curing.
The method in Patent Literature 3, however, is for clearly separating the functions of the at least two functional layers to be formed. An optical film produced thereby has, for example, a structure with layers of which the refractive indexes are greatly different. As a result, an interfacial reflection occurs to tint the optical film or cause interference fringes in the interface between the substrate and a layer formed on the substrate, thereby decreasing the visibility of images.
Also when an optical film is produced by simultaneously laminating at least two layers as described above, bubbles may occur in the lower functional layer(s) to whiten the layer(s), or to decrease the refractive index to give an undesired refractive index.